Conventionally, at large scale water treatment facilities such as water purification plants, water purification processes are performed by: removing fine suspended matter by filtering water through a layer of filtration media such as filtration sand; then by disinfecting the filtered water with chlorine. As cleansing methods of the filtration media, surface cleansing, which washes the surface of a sand layer by hitting it with water sprayed from a nozzle, and backwash cleansing, which forces purified water into a filtration reservoir from a lower pressure compartment, thereby floating the filtration sand grains, causing them to rub against each other, are in common use. However, if the backwash cleansing is repeated over a long period of time, the water pressure thereof influences even a gravel layer, creating areas of different thickness in said layer, which is optimally flat and of an even thickness. In addition, the surface cleansing and backwash cleansing methods cannot cleanse the filtration media with satisfactory efficacy. Problems arise from repeated use over a period of time such as contaminant accumulation on the filtration media. In this case, it is necessary to perform a regeneration process, which involves: totally ceasing the operation of the filtration reservoir, removing the filtration media, cleansing the filtration media, and replacing the contaminated filtration media, which has been cleansed. However, the regeneration process is extremely costly, and as during said process the filtration reservoir is not operating, it leads to a decrease in water treatment efficiency.
The applicant of the present invention, in order to meet this demand, has developed and proposed a sand cleansing apparatus which cleanses polluted filtration media in a shorter time and with a higher degree of cleansing ability (Japanese Unexamined Patent Publication Nos. 10(1998)-109051 and 11 (1999)-057526). This sand cleansing apparatus comprises: a sand receiving opening for receiving filtration media drawn from a filtration reservoir in its upper portion; a cleansing tank which stores sand and cleansing water having a sand extraction opening; a agitation tank erected within said cleansing tank having openings on the upper and lower ends thereof; and a screw conveyor which rotates within said agitation tank. The grains of sand are brought upward by the screw conveyor along with the cleansing water. As they are being conveyed upward, said grains of sand rub against each other by being agitated, and the scrubbing action thereof effectively removes the contaminants that are attached or coated thereon.
Filtration apparatuses, such as filtration tanks, which are installed in small scale simple plumbing networks or factories, differ from the large scale water purification plant described above. It is not practical to employ the sand cleansing apparatuses proposed by the present applicants (Japanese Unexamined Patent Publication Nos. 10(1998)-109051 and 11(1999)-057526) in these filtration apparatuses. This is because the utilization scales of the filtration apparatuses are small. Therefore, it is not efficient to secure space to install the sand cleansing apparatus in, and to expend costs associated with the installation and removal of the sand cleansing apparatus, when compared to the case of the filtration reservoir.
There are known filtration devices, as disclosed in Japanese Patent No. 31491 and Japanese Unexamined Utility Model Publication No. 63(1988)-98704, which have been developed in view of these points. The filtration device disclosed in Japanese Patent No. 31491 comprises a central pipe (cleansing tank), which is suspended by a frame (support portion). The lower end of the central pipe opens within a filtration chamber (filtration tank). A propeller is provided within the interior of the central pipe, at its lower end. A pipe having a jet expulsion opening is provided above the propeller, the jet expulsion opening being slightly above the upper edge of the central pipe. The pipe having the jet expulsion opening is linked to the propeller, and rotates therewith, to spray cleansing fluid in the horizontal direction by use of centrifugal force. During filtration, water having contaminants is supplied from above, and is filtered by passing through filtration sand, which is provided on an apertured false bottom (filtration floor). During cleansing, the propeller is rotated, causing filtration sand to be suctioned into the central pipe via the lower opening thereof. The filtration sand is elevated through the central pipe, then discharged in the horizontal direction by the cleansing fluid being sprayed from the jet expulsion opening. The filtration sand is cleansed by contaminants being separated therefrom at this time.
The filtration device disclosed in Japanese Unexamined Utility Model Publication No. 63-98704 comprises an elevating pipe (cleansing tank) erected therein, and a spiral water elevator provided within the elevating pipe. During filtration, water is discharged by a water dispersing tube, which is within filtration sand. Processed water, which has been filtered by passing through the filtration sand from below, is discharged above the filtration sand. During cleansing of the filtration sand, the spiral water elevator rotates and elevates the filtration sand, which has trapped contaminants, from the lower portion of the spiral water elevator. The contaminants are separated from the filtration sand by use of centrifugal force. The filtration sand is expelled through a filtration sand expulsion opening, which is provided in the upper portion of the elevating pipe.
Wear of the aforementioned propeller, central pipe, spiral water elevator and the elevating pipe is unavoidable over long term use, due to the relative movement among them and the filtration sand. Accordingly, the frequency of part replacement in the aforementioned devices is high.
In the filtration device disclosed in Japanese Patent No. 31491, the frame, to which the central pipe (cleansing tank) for cleansing the filtration media and the propeller is mounted, is suspended from a conduit pipe, which is provided at the upper portion of the filtration tank. However, the central pipe has a diameter far greater than the frame, and it cannot be dismounted from the conduit pipe to the exterior of the filtration tank. Accordingly, when the propeller or the central pipe become worn by the filtration media, it becomes necessary to disassemble the filtration tank to remove the propeller and the cleansing tank. In addition, filtration cannot be sustained, because agitation of the filtration media by the propeller crushes the filtration media (the properties of the filtration sand are changed).
In the filtration device disclosed in Japanese Unexamined Utility Model Publication No. 63-98704, the spiral water elevator is provided in an elevating pipe, which is formed integrally with a expulsion spout, through which filtered water is expelled. However, the elevating pipe is not configured to be removable from within a tank (filtration tank) In addition, the spiral water elevator is axially supported by small openings above and below it, and is also of a configuration that does not enable removal from within the tank. For these reasons, it is extremely difficult to exchange the elevating pipe and the spiral water elevator.